A reddish pink substance on heating gives off a vapour which condenses on the sides of the test tube and the substance turns blue. It on cooling, water is added to the residue then, it turns to its original colour. The substance is:

Options

A. Iodine crystals
B. Copper sulphate crystals
C. Cobalt chloride crystals
D. Zinc oxide

Answer

To solve the problem step by step, we need to analyze the information provided in the question and the characteristics of the substances given.

### Step 1: Identify the Characteristics of the Substance
The question states that we have a reddish pink substance that changes color upon heating. The color change is significant, as it indicates a transformation in the chemical structure or hydration state of the substance.

**Hint:** Look for substances that are known to change color when heated or when water is added.

### Step 2: Analyze the Given Substances
We have four substances to consider:
1. Iodine crystals - Black color
2. Copper sulfate crystals - Blue color
3. Cobalt chloride crystals - Reddish pink (magenta) color
4. Zinc oxide - White color

From this, we can see that the only reddish pink substance is cobalt chloride.

**Hint:** Eliminate substances that do not match the reddish pink color.

### Step 3: Understand the Heating Process
When cobalt chloride (CoCl2.6H2O) is heated, it loses water and becomes anhydrous cobalt chloride (CoCl2), which is blue in color. This matches the description in the question where the substance turns blue upon heating.

**Hint:** Consider the hydration state of cobalt chloride and how it changes with temperature.

### Step 4: Analyze the Cooling and Water Addition
Upon cooling, if water is added to the blue anhydrous cobalt chloride, it rehydrates to form cobalt chloride hexahydrate (CoCl2.6H2O), which is reddish pink again. This process explains the color change back to the original reddish pink when water is added.

**Hint:** Think about the reversibility of hydration reactions and how they affect color.

### Step 5: Conclusion
Based on the analysis, the substance that fits all the criteria described in the question is cobalt chloride crystals (CoCl2.6H2O).

**Final Answer:** The substance is cobalt chloride crystals.

### Summary of Steps:
1. Identify the color change of the substance upon heating.
2. Analyze the given substances and eliminate those that do not match the color.
3. Understand the heating process and the color change of cobalt chloride.
4. Consider the effect of cooling and adding water to the residue.
5. Conclude that the substance is cobalt chloride crystals.